effects of hydrothermal solutions on syenogranite, abu harba a...

IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 12, December 2015.

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Effects of Hydrothermal Solutions on Syenogranite, Abu Harba Area, North Eastern Desert, Egypt.

Osama, M. Draz and Ramag, A. Osman Nuclear Material authority, Cairo, Egypt.

Abstract

The area is located between latitudes 27⁰ 16` and 27⁰ 22` N and longitudes 33⁰ 06` and 33⁰ 14` E, in the north Eastern Desert of Egypt. Abu Harba area granites mainly composed of syenogranite, which have alkaline suite. Syenogranite is cut by sets of faults in addition to obvious network microfissures and joints that play a role of pathways to fluid flows. These structures dominate in the syenogranite and are causing effects of hydrothermal alterations (kaolinization and silicification), however the syenogranite still preserve its original texture. Violet fluorite mineral occurs in the syenogranite as disseminated large crystals with micro-crystals of phosinaite as secondary mineral in fluorite crystals in addition to the zircon and magnetite crystals. Abu Harba granites are display characteristics of A-type granitoids as a post subduction reduced peraluminous syenogranite. It has a high content of SiO2, alkalis with low Al2O3, CaO, MgO, TiO2 FeO, and Fe2O3 contents. It has a low, FeOt/MgO and K/Na ratios with reduced peraluminous nature. The syenogranite probably reflects the end product of extreme fractionation environment. The altered syenogranite show relative abundance of Rb, Zr, Zn and Y elements. zircon crystals usually occurs as solitary (monocrystal) and/or zoned aggregates crystals habit. Keywords: Fluorite, hydrothermal alterations, syenogranite, Egypt.

1-Introduction:

Abu Harba area is located in the North Eastern Desert of Egypt, northwest of Hurghada City (Fig. 1). Field and petrographic observations showed the first insights on the dynamic processes involved in the generation of the granitoid facies. Syenogranite is cut by sets of faults in addition to the observed network of microfissures and joints which play a role of pathways to fluid flows. Abu Harba granites are mainly composed of syenogranite, which have alkaline suite which still preserve their original texture in spite of the effects of hydrothermal alterations. Abd El Aty, (2013) revealed that the syenogranite of Abu Harba and Al Shagola have the chemical characteristics of calc-alkalic to alkalic calcic and metaluminous to slightly peraluminous rocks. It was generated and

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emplaced in post orogenic environment and described as highly fractionated I-type granites. The objectives of the present work are to discuss the role of hydrothermal solutions and their effects on the syenogranite rocks. In order to attain these purposes 12 representative samples were collected. These objectives were achieved by studying the mineralogical, petrographical and geochemical characters of the representative collected samples of the study area.

2-Geologic Setting:

The studied area is a mountainous region in the north Eastern Desert of Egypt between latitudes 27⁰ 16` and 27⁰ 22` N and longitudes 33⁰ 06` and 33⁰ 14` E. (Fig. 2). Syenogranite rocks in the studied area are coarse-grained and porphyritic with pink to red colors but turn to brownish red when stained with hematite along joints and fractures and speckled with many xenoliths. Syenogranite rocks are cut by one or more sets of faults mostly directed to the NE-SW. The contact between syenogranite and the alkali feldspar granite type especially at the western parts is gradational. On the other hand, this granite intrudes the Dokhan volcanics at its eastern and southern boundaries with irregular contacts.

Fig. 1: Location map of Abu Harba area, North Eastern Desert,

Egypt.

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Abu Harba granites altered in some sheared parts due to hydrothermal processes, especially along the fault planes and contacts. The most common alteration features are, kaolinitization, silicification sericitization and chloritization as well as iron and manganese oxides. Syenogranite is intensely crossed by dyke swarms which are mainly of basic composition. These dykes occur as long ribbons and ridges responsible for the many long narrow parallel ridges seen in the area. They are mostly following the NE-SW and ENE-WSW directions (Abd El Aty, 2013). 3-Sampling and Analytical Techniques: This work investigates 12 samples. All these samples represented a syenogranite suite (Fig. 2). Six samples were subjected to investigate the petrography and mineralogy characteristics.

Fig. 2: Geologic map of Abu Harba area, north Eastern Desert (Abd El Aty, 2013).

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Eight samples are selected to describe the chemical and radiometric nature of the studied granite. The major oxides were determined using the atomic absorption and wet chemical techniques, while the trace elements were analyzed using the X-ray fluorescence. eU and eTh contents were determined radiometrically using gamma-ray spectrometer; also U and Th was chemically determined using a spectrophotometer analysis. Radiometric investigated are prepared for crushing and separating grains, which were then hand-picked under a binocular microscope. The mineralogical identification was achieved by X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM) from Chemical War Laboratory on the separated grains. 4- Petrography Microscopic investigation on the selective representative studied fresh and altered samples were carried out to study the mineralogical composition, textures and the effects of hydrothermal solutions on these samples. Syenogranite exhibits hypidiomorphic granular texture. Some samples are porphyritic with orthoclase phenocrysts. They are mostly medium but some-times coarse-grained. They are mainly composed of K-feldspars (orthoclase and microcline with coarse patchy string and ripple perthites) and quartz (undulose extinction due to strain or deformation), with subordinate plagioclase (An 10–18), having carlsbad and albite twining) (Fig. 3-1). Microcline crystals are microfractured due to strain effects (Fig. 3-2). Biotite crystals have generally green brownish colors (Fig 3-3). Few of plagioclase crystals show bended subhedral to anhedral flaky crystals due to deformation (Fig. 3-4). Microcline has a tartan twinning (Fig. 3-5).

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Fig. 3: Photomicrographs of characteristic features. 3-1: Plagioclase crystals with albite twinning. 3-2: Microfractures in microcline crystal.

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3-3: Biotite crystal with chlorite crystal. 3-4: Subhedral zircon crystal in feldspar. 3-5: Tartan twinning in microcline. 3-6: Violet fluorite crystal with clay materials. 3-7: Plagioclase with mottled extinction in core. 3-8: Sericitization (small birefringent needles of potassic-muscovite) in plagioclase crystal (sieve texture), the feldspar crystal around, have perthitic texture. 5- GEOCHEMISTRY

5A- Geochemical characteristics Generally, the present granite display characteristics of A-type which

have high contents of SiO2, alkalis (Na2O+K2O) wt %, Rb, Nb, Y and Zr. With low contents of Al2O3, CaO, MgO, TiO2, FeO, Fe2O3, Sr and Ni. It has low FeOt/MgO and K/Na ratios, Table (1&2). Table 1: Major oxides (wt %), and trace elements (ppm) analysis for Abu Harba area granites.

Major oxides

Oxides 1 2 3 4 5 6 7 8 Av. SiO2 73.9 74.01 74.06 73.11 73.50 73.55 73.00 72.92 73.50 TiO2 0.11 0.12 0.11 0.10 0.11 0.13 0.15 0.15 0.12

Al2O3 12.85 13.10 12.82 12.97 13.23 13.10 13.11 13.37 13.06 Fe2O3 0.66 0.64 0.67 1.28 0.81 0.85 1.11 1.08 0.88 FeO 1.98 1.79 1.99 1.95 2.09 1.75 1.95 1.81 1.91 MnO 0.06 0.06 0.07 0.08 0.08 0.08 0.09 0.09 0.07 MgO 0.71 0.54 0.65 0.78 0.55 0.62 0.82 0.8 0.68 CaO 0.86 0.91 0.86 0.90 0.88 0.80 1.11 0.98 0.91 Na2O 3.79 3.81 3.86 3.80 3.80 3.76 3.74 3.70 3.78 K2O 4.00 3.98 4.02 4.02 3.96 3.99 3.91 4.00 3.98 P2O5 0.05 0.14 0.18 0.19 0.15 0.18 0.15 0.20 0.15 L.O.I 0.23 0.24 0.15 0.65 0.40 0.57 0.69 0.67 0.45 Total 99.22 99.34 99.47 99.83 99.56 99.38 99.73 99.87 99.55

Trace elements Cr 16 39 17 13 15 12 14 17 18 Co 3 4 3 4 4 4 4 4 4 Ni 11 11 12 10 13 14 14 13 11 Cu 11 12 12 11 12 11 12 12 11 Zn 25 35 26 38 40 50 35 44 36 Zr 160 155 145 153 165 165 154 165 157 Rb 292 265 305 283 249 239 250 233 264 Y 57 61 59 55 59 66 65 60 60 Ba 160 173 140 200 170 230 194 175 180 Pb 8 6 7 8 5 11 6 8 7 Sr 40 45 38 45 44 50 45 41 44 Ga 23 22 24 21 25 20 23 22 22 Nb 35 33 35 30 33 29 33 25 31 U 7 8 6 7 6 8 5 6 6

Th 16 16 15 12 16 17 15 14 15

Table (2): Some geochemical ratios of the studied granites.

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1 2 3 4 5 6 7 8 Av. K/Na (Mol.) 0.69 0.68 0.68 0.69 0.68 0.69 0.68 0.71 0.68 FeOt/MgO 3.62 4.38 3.98 3.97 5.12 4.01 3.27 3.47 3.97

Ga/Al 3.40 3.19 3.55 3.07 3.59 2.85 3.33 3.12 3.26 Y/Nb 1.62 1.84 1.68 1.83 1.78 2.53 1.96 2.40 1.95 D.I. 89.25 89.89 90.08 88.48 89.16 89.47 87.5 87.94 88.79

The average of whole-rock major composition of the selected analyzed granites, are

listed on table (3). The average of the studied granite are compared with those of

comparable Egyptian and world granites, indicate it has comparable the studied

granites display comparable values of SiO2, Al2O3, F2Ot, MnO and P2O5, While it

has lower ∑ Alkalies and higher value in MgO, than the rest of the granites (Table 3).

Table (3): Comparison of the average compostion of Abu Harba area granites with other comparable grantoids. References (Average)

1 2 3 4 5 6

SiO2 73.20 74.27 73.81 73.38 70.60 76.34 TiO2 0.12 0.20 0.26 0.30 0.39 76.34

Al2O3 13.09 13.61 12.40 12.88 14.08 12.13 Fe2O3 1.24 - 1.24 - 2.82 1.72 FeO 1.59 2.03 1.58 2.44 - - MnO 0.07 0.05 0.06 0.06 0.11 0.07 MgO 0.73 0.27 0.20 0.30 0.58 0.14 CaO 0.93 0.71 0.75 1.06 1.43 0.33 Na2O 3.77 3.48 4.07 3.50 4.38 4.09 K2O 3.97 5.06 4.65 4.62 4.28 4.57

∑ Alkalies 7.75 8.54 8.72 8.12 8.66 8.66 P2O5 0.16 0.14 0.04 0.07 0.11 0.04

1- The present Abu Harba area A- Type granite. 2- Low –Ca granite (Turekian & Wedepohl, 1961). 3- A - Type granite (Whalen et al., 1987). 4- A- Type granite (Chappel & White, 1992). 5- Earlier phase of Abu Harba granite (M. Roz, 2005). 6- Later phase of Abu Harba granite (M. Roz, 2005).

5B - Chemical Classification

Middlemost (1985) proposed a classification of igneous rocks based on the

binary relation between (Na2O+k2O) and SiO2 (wt.%), as shown in Fig.(4). This

diagram display that Abu Harba area granites fall within the granite field, and

confirmed in more specific nature which show as a syenogranite in the ternary

discriminated Or-Ab-An diagram (Streckeisen, 1976b) Fig. (5).

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Fig. (4): Chemical classification of the studied granite (after Middlemost, 1985).

Fig. (5); The studied granite plotted on the Or-Ab-An ternary diagram (after

Streckiesen, 1976a).

5-C Geochemical variation and magmatic type. Geochemical variation

Some major oxides and trace elements are plotted in Fig. (6). It is noticed from these

variation diagrams that; Abu Harba area granites having a narrow D.I. range values

from 87.5 to 90.08 with average 88.79, reflected a homogeneous nature of Abu Harba

area. Generally, It having a high SiO2, ∑(K2O, Na2O), Rb, Zr, Nb and Y, moderately

MgO, Al2O3, while they possess lower values in TiO2, FeOt, MgO, CaO, P2O5 and

Sr. D.I, values display correlate positively with SiO2, K2O, Na2O, Rb, Nb, linear

correlation with Zr, while it shows negative correlation with Al2O3, TiO2, FeOt,

MgO,

CaO, Zr, Pb, Ni, Sr and Y.

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Fig 6: Some major oxides and trace elements against D.I. values of Abu Harba granites.

12.7

12.9

13.1

13.3

13.5

87 88 89 90

D.I.

Al2O

3

72.8

73.2

73.6

74

74.4

87 88 89 90

D.I.

SiO

2

2.2

2.4

2.6

2.8

3

3.2

87 88 89 90D.I.

FeO

t

3.65

3.75

3.85

87 88 89 90

D.I.

Na2O

0.7

0.8

0.9

1

1.1

1.2

87 88 89 90

D.I.

CaO

3.85

3.95

4.05

87 88 89 90

D.I.

K2O

200

240

280

320

87 88 89 90

D.I.

Rb

20

25

30

35

40

87 88 89 90

D.I.

Nb

52

56

60

64

68

87 87.5 88 88.5 89 89.5 90 90.5D.I.

Y

35

40

45

50

55

87 88 89 90 91

D.I.

Sr

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Magma Series The investigated granites are display an alkaline nature in which are documented with

the following discriminated diagrams. Plots of the investigated granites on the

Wright's alkalinity ratio diagram show that the all eight samples fall in the strong

alkaline fields by plotting Si02 versus Log (K20/MgO). Maniar and Piccoli (1989)

using the molar A/NK and A/CNK ratios to discriminate between granitoid rocks, and

show peraluminous nature among the studied granitic samples Fig. (7). On plotting

Abu Harba granitic samples on the discriminated diagram, (after Whalen et al., 1987),

and later Hong et al., (1996) delineated the (AA) anorogenic and (PA) post orogenic

fields, it fall in the A-type granite and post orogenic type Fig (8). In recent years of

the past century, there has tendency to subdivide the A-type granites into two major

clans: post orogenic and anorogenic types. Eby (1992) subdivided A-type

Fig. (7): The studied granites plot on the SiO2 versus Log K2O/MgO diagram (after Rogers and Greenberg, 1981). Fig. (8) The studied granite plot on the ACNK-ANK diagram (after Maniar and Picolli, 1989b). granites into A1 and A2 groups. The A1 group refers to differentiates of magmas derived

from sources like those of oceanic–island basalts but emplaced in continental rifts or during

intraplate magmatism. The A2 group, on the other hand represents magma derived from

continental crust or under plated crust that has been subjected through a cycle of

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Fig. (9): The studied granite plotted on the 10000Ga/Al versus some trace element

(Whalen et al., 1987), AA, PA fields redrawn after Hong et al., (1996).

Fig. (10): The studied granite plotted on the Y-Nb-3Ga ternary diagram (after Eby, 1992).

continent-continent collision or island-arc magmatism (Eby 1992)Fig. 10. On plotting

the studied granitic samples, they fall totally on the post orogenic with Y/Nb ratios

greater than 1.2 on the (Nb-Y-3Ga) diagram Fig. (9). Recently, a few useful

discrimination diagrams were designed by (El Dabe 2015), and employed successfully

to optimal subdivision among various A-type according to their magma and tectonic

setting regimes used ''element screens" such as Y/Nb, K/Na (Mol), FeOt/MgO ratios

and differentiation index values (D.I.). El Dabe (2015), suggest that the post orogenic

and anorogenic A-type granitiods, can be further classified into the four known

following types:, Hot spot (HSR), and rift (RRG) related to the anorogenic type, while

the post subduction ( PSRG) and post continent collisions (PCCCRG) are belong to

the post orogenic alkaline granitoids. He (op.cit) add that both of rift and post

subductin groups have peralumnous, metaluminous and peralkaline varieties, while

the hot spote and post continent- continent collision groups possess only

metaluminous and peralkaline varieties. On the Y/Nb and K/Na (Mol) binary diagram

El Dabe (2015), the investigated samples plot close to post subduction alkaline granite

field (PSRG) (Figs. 11&12) on the overlap

Zr

10000 Ga/Al

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Fig. (11): The studied granite plotted on the Y/Nb and K/Na (Mol) binary

diagram (after El Dabe, 2015).

Fig. (12): The studied granite plotted on the Y/Nb and FeOt/MgO diagram

binary diagram (after El Dabe, 2015).

Fig. (13): The studied granite plotted on the D.I., FeOt/MgO and 100Y/Nb ternary


Fig. (14): The studied granite plotted on the on Y and Nb diagram (Pearce et al.,

1984).

area with the post continent –continent granites field, due to low (K2O) values, while

it fall totally on the post subduction alkaline granite field (PSRG), due to its low

FeOt/MgO values on the Y/Nb and FeOt/MgO diagram. Finally, Abu Harba granites

fall totally on the post subduction alkaline magma type field (PSRG) and low D.I.

value zone on the most discriminated D.I., FeOt/MgO and 100Y/Nb ternary diagram

)

b

O

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Figs. (10&11&12&13), due to its high Y/Nb have values lesser than 4, accompanied

with low D.I. and FeOt/MgO values El Dabe (2015).

5D-Tectonic setting and tectonic setting regimes Generally, all A-type granitiods should located on the within plate field (Pearce et all

1984), Abu Harba granitic samples plot in the within plate field, on Y+Nb and Rb

diagram (Pearce et al., 1984) Fig. (14). Recently, El Dabe (2015), employed Rb, 10

FeOt/MgO and Y+Nb diagram to distinguished among the four hot spot, rift, post

subduction and post continent- continent collision A-types. Plotting Abu Harba

granites straddle on the boundary line between post subduction (PSRG) and rift

(RRG) type fields Fig. (15). To demonstrate the regime in which associated with the

A-type granite magma and tectonic types, El Dabe (2015) suggest that both of rift and

post subductin groups have both reduction and oxidation tectonic regimes,

Fig. (15): The studied granite plotted on the on Rb, 10 FeOt/MgO and Y+Nb


Fig. (16): The studied granite plotted on the on Y+Nb/10, 10Fe2O3 and 5 Fe2O

ternary diagrams (after El Dabe, 2015).

due to their involved reduced peraluminous and oxidized metaluminous, and

peralkaline varieties, alternatively, while the hot spot and post continent- continent

collision types possess only oxidized regime, due to their involving oxidized

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metaluminous and peralkaline varieties. Most of investigated samples display slightly

reduced peraluminous regime, except three samples are fall in the oxidized condition

regime, due to relatively high FeO values on the Y+Nb/10, 10Fe2O3 and 5Fe2O

ternary diagram Fig. (16).

6-Radioactivity: Generally, the same samples which chemically analyzed are subjected to the

radiometric analyses to determine their uranium and thorium contents Table (1 & 4 ).

Abu Harba area granites contains a narrow range values of chemical uranium (Uc)

ranging from 5 ppm to 8 ppm with an average values of 6 ppm, and Th content

ranging from 14 ppm to 24 ppm with an average values of 15 ppm. The Th /U ratios

range from 1.70 to 3.00. Abu Harba area granites posses eU content ranging from 7

ppm to 11 ppm , with an average of 9 and eTh content ranging from 14 to 24 , with an

average of 18 ppm . Equivalent Th/U ratios values, have a rang varying from 1.55 to 3

ppm. The obviously gap between the (chemical and equivalent) U and Th contents

indicating a high degree of disequilibrium state which describes the environments

enclosing both U, Th and the hydrothermal solutions associated the studied granite

(Table 4). The disequilibrium stat was described with the equilibrium factor (D),

which is expressed by the ratios of chemically analyzed uranium (Uc) over

radiometrically (eU) , i.e. D-factor + Uc/Ur (Hansink, 1976). If the D-factor is higher

or lower than unity, then addition or removal of uranium has occurred, respectively.

The calculated (D) factor with an average of 0.72 is indicating removal of uranium

from the studied granites, and its concentration probably along fracture zones. Atwiya

(1984) mentioned that uranium could be released from the granite itself by dissolution

of accessory uranium- bearing minerals and redeposition in fractures and shear zones

by percolating solutions. According to the inter elements relationship, the following

variation diagrams describe the behaviors of both uranium and thorium element with

the other elements. On plotting the Uc content versus Thc values, show a clear

positive correlation with each other, indicating that U and Th distribution were

essentially controlled by the magmatic processes control the exist the two elements

Fig. ( 17 ).

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Table: (4) Chemical and radiometric uranium and thorium contents of the studied granites.

Serial Chemical analysis Radiometric analysis D- factor Uc Thc cTh/U eU eTh eTh/U

1 7 16 2.28 9 16 1.77 0.77 2 8 16 2.00 11 14 1.27 0.72 3 6 15 2.50 6 18 3.00 1.00 4 7 12 1.70 11 21 1.91 0.63 5 6 16 2.66 7 22 3.14 0.85 6 8 17 2.10 10 24 2.40 0.80 7 5 15 3.00 9 14 1.55 0.55 8 6 14 2.33 8 17 2.12 0.75

Av. 6 15 9.00 9 18 2.00 0.75

While plotting the Uc content against Zr Fig.(18), shows a moderately positive

correlation with some degree of scattering, suggesting leaching and redistribution of

uranium probably under the influence of hydrothermal solutions. The same result is

consistent with plotting the Uc versus the P2O5 Fig. (19), which display almost

slightly linear trend with scattered pattern, due to the effect of the post magmatic

processes.

Fig. (17): Uc content versus Thc content variation diagram for the studied granites. Fig. (18): Uc content versus Zr content versus Thc variation diagram for the studied granites.

10

12

14

16

18

20

4 5 6 7 8 9

(Uc)

(Th

c)

140

150

160

170

4 5 6 7 8 9

UC

Zr

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0.1

0.15

0.2

0.25

4 5 6 7 8 9Uc

P2O

3

Fig. (19): Uc content versus P2O5 content variation diagram for the studied granite

7- Mineralogy Mineralogical identification was done by using binuclear microscope, X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM). The ESEM techniques are used on the separated grains from the anomalous samples in the syeogranite. Scanning electron microscope data indicate that zircon crystals usually occurs as solitary (monocrystal) and/or zoned aggregates crystals habit. Microparticles of zircon occurr in fluorite crystal as inclusions. These microparticles are crystalline calcium phosphosilicate formed as ghost mirmaktic texture in fluorite crystals. However the crystal structure of fluorite permits incorporation of many genetically important trace and rare-earth elements (Möller et al. 1998). These nanocrystalline mineral is phosinaite (Figs. 20&21).

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Fig 20: Images of selected fluorite crystals separated from studied syenogranite

20-1: Image of violet fluorite grains. 20-1-a: XRD of fluorite. Abbreviation: 20-1-b: XRD of fluorite after heating. F: Fluorite, H: hematite, P: Phosinate. 20-2: Image of zircon crystals. 20-3-: Image of bipyramidal magnetite crystals.

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Fig 21: ESEM images for distinct morphological forms of zircon crystals separated from studied syenogranite Fig 21-a: solitary zircon (monocrystal). Fig 21-b: Twinning zircon (aggregates crystals). Fig 21-c: Anhedral fluorite grain. Fig 21-d: Phosinate nanomineral association with fluorite grain as inclusion. Fig 21-e: Bipyramidal magnetite crystal. Altered granites Field studies indicated that the Abu Harba granites have suffered from hydrothermal alterations, especially along faults and fractures zones. The fresh granite is usually characterized by its pink to slight reddish pink color, while the lighter tones such as creamy and brownish yellow colors could be attributed to the destruction of feldspars by kaolinization. The silicified granite is characterized by its hardness and lighter rosy tones. From the above description, the alteration processes that affected the studied rocks could be classified into two main types in decreasing order of predominance as follow:- kaolinization and silicification. The different alteration features, in the study area, are always strongly associated with shear zones. Some alteration features are also noticed such as chloritization, and epidotization, but are of less importance of except fluoritization besides the presence of Mn dendrites. The presence of fluoritization mainly associated with uranium mineralization indicates that the alteration processes are mainly due to hydrothermal activity. Samples representing the main two types of alterations were collected separately and analyzed for major oxides and trace elements (table 5), in order to study their geochemical behavior. The altered samples are plotted on the AKF ternary diagram after (Meyer and Hemely,1967) (Fig. 22) where A=Al2O3, K=k2O+Na2O, and F=FeOt + MnO +MgO. They show that, all samples fall in sericite facies. On the (Na2O+CaO)- Al2O3- K2O ternary diagram (Nesbitt and young, 1989), (Fig. 23) all samples plot

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parallel to advanced weathering trend. The initial stages of weathering from a trend parallel to the (Na2O+CaO)-Al2O3 side of the diagram, where as advanced weathering shows a market loss in K2O and CaO+ Na2O compositions and moves more towards the Al2O3 apex. Accordingly, all-previous diagrams, revealed that the alteration processes in Abu Harba granites are mainly due to hydrothermal activity. Silicification process The silicified granites in Abu Harba area are characterized by their creamy color and light tone as their silica content could reach as much as 80%. Exley (1976), jasinski (1988) and pier et al.(1992) reported that SiO2 could develop due the hydration of feldspars as follows :-

0.75Na2CaAl4Si8O24 +2H++ k+ KAl3Si3O10(OH)2 +1.5Na+ + 0.75Ca+2 +3SiO2

Plagioclase Muscovite

1.5 KAlSi3O8 + H+ 0.5 KAl3Si3O10 (OH)2 + K+ + 3SiO2 Potash feldspar muscovite Bucanan (1982 ) stated that the silicification process may result from strong acidic hydrothermal solutions with temperatures varying between 300o and 400oC and a pH of 1-3. The silicification process results in an increase of SiO2, TiO2,P2O5 and L.O.I. at the expense of the other major oxides .This is always accompanied with the increase of some trace elements such as Zr,Y,Nb,Hf,Ta,Pb and F. Kaolinization process Helgeson (1974) stated that the presence of kaolinite indicates that the rocks were affected by strong acidic solution at PH from 2 to 3 and low temperatures varying between 200o and 250o C. He added that kaolinite is not stable with quartz at temperatures above 300o C. The temperature of the hydrothermal solutions responsible for kaolinization could have varied between 150o and 250o C. Rice (1973) pointed out that kaolinite may be formed in granite due to hydrolysis of albite. In this reaction, the silica precipitates as an amorphous phase.

4NaAlSi3O6 + 4H2CO3 + 18H2O 4Na+ + 4HCO3- + 8H4SiO4 + Al4Si4O11

Albite kaolinite

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The kaolinization process causes an increase an of TiO2, P2O5 and L.O.L. at the expense of the other major oxides .This is always associated with an increase of some trace elements such as Ba,Zr,Hf,Ta,Pb and F. This enrichment may be referred to the alteration of feldspars and mica or could be due to the effect of hydrothermal activity. Table 5: Major elements (oxides wt %) and trace elements for studied altered granites.

Rock type

Silicified granites Kaolinized granites

Sample No

1 2 3 4 5 6 7 8 9 10

SiO2 78.90 79.60 78.77 79.26 79.55 73.55 74.02 73.71 72.99 73.20 TiO2 0.13 0.14 0.13 0.14 0.13 0.11 0.11 0.13 0.11 0.14

Al2O3 12.04 12.77 12.23 12.45 12.30 13.66 13.45 13.47 13.53 13.69 Fe2O3 0.65 0.59 0.61 0.57 0.57 0.82 0.79 0.80 0.82 0.83 FeO 0.60 0.58 0.60 0.62 0.60 0.44 0.42 0.43 0.43 0.44 MnO 0.03 0.04 0.04 0.03 0.04 0.03 0.02 0.03 0.03 0.03 MgO 0.24 0.25 0.24 0.23 0.22 0.21 0.22 0.20 0.23 0.23 CaO 0.33 0.32 0.32 0.31 0.32 0.50 0.51 0.48 0.52 0.50 Na2O 2.49 2.33 2.47 2.40 2.42 3.63 3.60 3.59 3.63 3.57 K2O 2.70 2.45 2.66 2.52 2.63 3.64 3.60 3.57 3.62 3.57 P2O5 0.14 0.12 0.12 0.13 0.12 0.14 0.13 0.14 0.14 0.14 L.O.I. 1.60 1.18 1.35 1.22 1.21 3.40 3.11 3.36 3.55 3.48 Total 99.85 100.37 99.54 99.88 100.11 100.13 99.98 99.91 99.60 99.82

Trace elements Rb 195 200 199 196 211 222 230 199 221 209 Ba 29 22 26 25 30 56 58 46 50 48 Sr 8 7 7 9 7 5 8 7 9 10 Zr 170 185 177 180 184 188 170 190 176 187 Y 110 99 100 103 111 86 88 85 79 88

Nb 86 98 95 90 83 60 63 66 70 69 Ga 21 22 22 21 24 25 25 22 27 25 Hf 15 15 14 14 15 15 15 13 14 15 Ta 10 10 11 10 11 7 7 9 10 8 Zn 50 55 54 51 50 72 76 77 70 66 Pb 33 40 35 42 39 44 46 47 43 55 F 20 22 24 20 25 26 23 21 22 26

Radioactivity of altered granites The studied radioactive occurrence in Abu Harba area associated with strongly altered granites, mainly characterized by their intense, kaolinization and silicification .Table (6) shows the eU, eTh contents and eTh/eU ratio of the altered granites of Abu Harba area.

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Table (6): eU, eTh, eTh/eU distribution in the altered granites

Rock type Sample No.

eU eTh eTh/eU

Silicified granites

1 35 22 0.62 2 31 21 0.67 3 25 19 0.76 4 39 24 0.62 5 44 27 0.62

Kaolinized

granites

6 36 23 0.63 7 42 24 0.57 8 33 40 1.21 9 40 23 0.58 10 51 30 0.59

In the silicified granite, the eU content ranges from 25 to 44 ppm and eTh content is between 19 and 27 ppm. The kaolinized granite, the eU content ranges from 33 to 51 ppm, while the eTh content varies from 23 to 40 ppm.. Generally, in the altered granites, the effect of hydrothermal solutions is obvious through the mobilization and redistribution of uranium by hydrothermal solutions during post magmatic processes.

Conclusion:

The detailed petrographic, mineralogical and chemical studies are applied on Abu Harba granite. The main objective of this study is to clear the role of hydrothermal solutions and its effect on Abu Harba granite. It represents a post subduction peraluminous alkaline biotite syenogranite. Subsolidus alterations are the main features of this syenogranite. The main accessory minerals are fluorite, zircon and magnetite. Two main types of alterations kaolinization and silicification. Some alteration features are also noticed such as chloritization, epidotization, and fluoritization. The presence of fluoritization mainly associated with uranium mineralization indicates that the alteration processes are mainly due to hydrothermal activity. The effect of hydrothermal solutions is obvious through the mobilization and redistribution of uranium by hydrothermal solutions during post magmatic processes. .

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Fig. 22: Al2O3-(K2O+Na2O)-(FeOt+MnO+MgO) ternary diagram (Meyer and Hemly, 1967) for altered granitic rocks

Fig. 23: Al2O3-(CaO+Na2O)-K2O ternary diagram (Nesbit and Young, 1989) for altered granitic rocks

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effects of hydrothermal solutions on syenogranite, abu harba a...

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